This section provides background information related to the present disclosure which is not necessarily prior art.
FIG. 1 is a basic block diagram of an example of a prior art uninterruptible power supply (“UPS”) system 100. UPS system 100 includes a rectifier/charger 102, a battery 104, a DC bus 106, an inverter 108, a bypass switch 110, a control module 112, and an output transformer 114. It should be understood that some UPS systems do not include an output transformer, and that the source of back-up DC power may be other than a battery, in which case rectifier/charger 102 would not include a charger. It should also be understood that rectifier/charger 102 may only include the rectifier and UPS system 100 have a separate charger. When UPS system 100 is operating in a double conversion mode, alternating current (“AC”) power is supplied at an input 116 of UPS system 100. Rectifier/charger 102 converts the incoming AC power to direct current (“DC”) power. This DC power is supplied to battery 104 to charge the battery. The DC power is also supplied to the DC bus 106 which is coupled to an input of inverter 108. Inverter 108 converts the DC power to AC output power that is then supplied to a load 118 via the output transformer 114. If the normal AC power source fails, battery 104 provides power to the DC bus 106 which is converted by inverter 108 to AC output power. In some cases, UPS system 100 is switched to a bypass mode where bypass switch 110 is closed. AC power then flows directly from input 116 to load 118 via output transformer 114 (or directly to load 118 in UPS systems not having an output transformer). UPS system 100 may be switched to the bypass mode when a component of UPS system 100 in the double conversion power flow path fails, such as rectifier/charger 102 or inverter 108, or when the quality of AC power source is sufficient that it can be used to directly power load 118 without being conditioned through the double conversion path.
Control module 112 controls the rectifier/charger 102, inverter 108, and bypass switch 110. Control module 112 monitors the input and output voltages and currents and controls the rectifier/charger 102 to charge the battery and regulate the DC power including the bus voltage and also controls inverter 108 to regulate the AC output power including the AC voltage.
A simple repetitive controller for UPS systems is a good controller for harmonic rejection with various loads, especially for nonlinear loads. But an inherent disadvantage of repetitive control is that it can't provide fast, sub-cycle response, which is one of the most important features of UPS systems to not only maintain a nice sinusoidal voltage, but also provide fast transient responses. Another disadvantage of repetitive control is that it's difficult to stabilize without sacrificing the steady state voltage performance.
Repetitive control is a control method specifically used in dealing with periodic signals. It uses the periodicity of the reference or disturbance to provide a good harmonic rejection. As it is apparent to those skilled in art, the discrete transfer function of a repetitive controller, is shown below in:RPC(Z)=Z^(−(N−k))/(1−[(Q(Z)·Z)]^(−N))*Kopt*S(Z)*Z^(−k)  (Equation 1. Discrete Transfer Function of a Repetitive Controller)where Q(Z) is a constant gain smaller than 1, N is the number of samples per cycle at a fixed sample rate, S(Z) is a compensator, and Kopt is the optimized gain for the best transient and steady state performance of the control loop. Z is the symbol for Z-transformation, Z=ejwt, where, w=2*π*T, T=1/fs is the sampling period and fs is the sampling rate. k is the kth number of all samples in a sampling period T.
One of the disadvantages of the repetitive controller is that it's difficult to stabilize with various types of load. The gain Q(Z) shown in Equation 1 above is the key to stabilizing a repetitive controller. Q(Z) must be smaller than 1 and the smaller the gain, the more stable the controller is with various types of load. However, the smaller the Q(Z) gain, the less accurate the steady state controller's performance is. So it's difficult to have a robust stable repetitive controller for various types of load without sacrificing the steady state controller's performance.
Another inherent disadvantage of the repetitive controller is that it can't provide fast, sub-cycle response, which is one of the most important feature of UPS systems. It's critical that a UPS system respond quickly at the time of a utility power outage and provide a fast transient response.